You'll have to edit /etc/inetd.conf manually and add 
lines like the following:

  pop3 stream tcp  nowait root ${PREFIX}/sbin/spop3d spop3d
  pop3 stream tcp6 nowait root ${PREFIX}/sbin/spop3d spop3d

The second line is only necessary if you want to use IPv6
support.

inetd(8) must be reloaded (or started). To enable it at boot, the
following line needs to be added to rc.conf.local(5):
inetd_flags=

An optional global configuration file may be used in ${SYSCONFDIR}
called spop3d.conf.

See spop3d(8) and spop3d.conf(5) for available options.
